Study Material for the Course: Characterization of Materials

Instructor:

Dr. Sanjay Kumar Vajpai,Assistant Professor,

Department of Metallurgical and Materials Engineering,

National Institute of Technology, Jamshedpur

Vibrational Spectroscopy: Fourier Transform Infra-Red (FTIR) and Raman

Vibrational Spectroscopy

• Analysis of the structure of molecules through the examination of the interaction between electromagnetic radiation and nuclear vibrations in molecules.

• Electromagnetic waves with wavelengths in the order of 10−7 m, typically Infrared light, is used because energies of infrared light match with vibrational energies of molecules.

• Molecular vibrations are detected by either the absorption of infrared light or the inelastic scattering of light by a molecule.

• It can be used for the examination of: gases, liquids, solids, and a variety of organic and inorganic substances.

• Since metallic materials strongly reflect electromagnetic waves, it cannot be used for metallic materials.

• Two most commonly used spectroscopic methods: Fourier transform infrared spectroscopy (FTIR) and Raman microscopy (also called micro-Raman)

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Basic Theory and BackgroundEnergy, frequency, wavelength, and wave number ranges of electromagnetic waves.

Note that Frequency range of molecular vibrations is in the infrared region close to visible light.3

Electromagnetic Spectrum

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Basic Theory and Background

• Electromagnetic Radiation (ER): Travel at the speed of light, varies in wavelength from radio

waves (∼102 m) to γ -rays (∼10-12 m).

• The energies of molecular vibrations match with those of electromagnetic radiation in a

wavelength range near visible light, i.e. in the range of wavelength of 0.40–0.75×10−6 m.

• The ER in near visible light can change the state of molecular vibrations and produce vibrational

spectra of molecules.

• Vibrational spectroscopy characterizes the electromagnetic waves in terms of wave number.• Wave number is defined as the reciprocal of wavelength (λ) in the units of cm−1 or proportional to

the frequency of the electromagnetic wave (ט) with a constant factor of the reciprocal of the speed

of light (c).

𝜐 =1

𝜆=𝜐

𝑐Wave number = reciprocal of wavelength ,

Wave number can also be understood as the number of waves in a 1 cm-long wave-train, and it

represents the energy of the radiation, similar to wavelength.

If we consider electromagnetic waves as Photons, the Photon energy can be represented in

terms of wave number as follows: 𝑬 = 𝒉𝝊 = 𝒉𝒄 𝝊5

Molecular Vibrations• In solids, the molecular vibrations can be defined as a periodic motion of the atoms of a molecule relative

to each other without altering the center of mass of the molecule. In fact the molecules always remain in vibration at all temperatures except at absolute zero.

• The typical vibrational frequencies: from 1012 Hz to approx. 1014 Hz, (i.e. wavenumber from 300 to 3000 cm−1.)

• For simplistic model of Molecular vibrations, it can be considered that the two nuclei in a diatomic molecule are connected by massless springs and the diatomic molecule vibrates by stretching or compressing the bond (a massless spring) between two nuclei. Such vibration motion is considered as harmonic, which is a good approximation for molecular vibrations with small displacement. However, there could be a small deviation for large vibrations.

For harmonic vibration, the vibrational energy can be described as 𝐸𝑣𝑖𝑏 = ℎ𝜈𝑣𝑖𝑏 𝜐 +1

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υ is the vibrational quantum number, which defines distinguishable vibrational levels and νvib is the vibrational frequency of a molecule.• The vibrational frequency of molecules is in the range of mid-infrared (IR) frequencies (6×1012 –

1.2×1014 Hz).• The ground state corresponds to υ =0 with corresponding vibrational energy of 1/2 hνvib; • The first excited level (υ =1) should have vibrational energy of 3/2 hνvib; and so on.

In general, the vibrational spectroscopy covers a wave number range from 200 to 4000 cm−1.6

Principles of Vibrational Spectroscopy: Infrared Absorption• Phenomenon of infrared absorption by molecular vibrations.• A molecule is irradiated by electromagnetic waves within the infrared frequency range.• If one particular frequency matches the vibrational frequency of the molecule (νvib), the

molecular vibration will be excited by waves with the frequency νph =νvib and theelectromagnetic radiations with the specific frequency νph will be absorbed by themolecule because the photon energy is transferred to excite molecular vibrations.

• The excitation means that the energy of molecular vibrations will increase, normally byυ=+1.

• The fundamental transition from υ =0 to υ =1 dominates the infrared absorption,although other transitions are also possible.

• For example, if the HCl diatomic molecule with νvib=8.67×1013 Hz is excited by thisfrequency of electromagnetic radiation, the intensity of radiation at that frequency(8.67×1013Hz) will be reduced (absorbed) in the infrared spectrum, while the moleculeitself will be moved to a higher vibrational energy level.

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Principles of Vibrational Spectroscopy: Infrared Absorption

Typical IR absorption spectrum of hexanal

An individual deep valley represents a single vibration band that corresponds to a certain

molecular vibration frequency.

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Principles of Vibrational Spectroscopy: Raman Scattering• Raman spectroscopy is based on the Raman scattering phenomenon of electromagnetic radiation by

molecules. • When a material is irradiated with electromagnetic radiation of single frequency, the light is scattered by

molecules both elastically and inelastically.• Elastic (Rayleigh) Scattering : No net energy transfer from photon to molecular vibration scattered light

has the same frequency as that of the incident radiation.• Inelastic (Raman) Scattering: different frequency of scattered light than that of the radiation scattered

photons have either lower or higher energy than the incident photons

𝐸𝑛𝑒𝑟𝑔𝑦 𝐶ℎ𝑎𝑛𝑔𝑒, ΔΕ𝑝ℎ ∝ 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦, ∆𝜈𝑝ℎ

Decreasing ∆𝜈𝑝ℎ means higher energy level of molecular vibrations Stokes Scattering.

Increasing ∆𝜈𝑝ℎ means lower energy level of molecular vibrations Anti-Stokes Scattering.

Generally, Raman spectrum records the frequency changes caused by the Stokes scattering by molecules.

∆𝝂𝒑𝒉 is called Raman Shift in the Spectrum

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Principles of Vibrational Spectroscopy: Raman ScatteringTypical Raman spectrum of polycrystalline graphite

• The intensity of the Raman shift is

plotted over a range of wave

numbers.

• An individual band of Raman shift

corresponds to a molecular vibration

frequency.

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Fourier Transform Infrared Spectroscopy (FTIR)• Most widely used vibrational spectroscopic technique.

• The Fourier transform method is used to obtain an infrared spectrum in a whole range of wave numbers

simultaneously, in contrast to dispersive method that relies on creating a spectrum by collecting signals at

each wave number separately.

• FTIR has a much higher signal-to-noise ratio than that of the dispersive method.

• FTIR analysis measures the range of wavelengths in the infrared region that are absorbed by a material.

• The sample of a material is irradiated by infrared radiation (IR) and the sample’s absorbance of the infrared

light’s energy at various wavelengths is measured to determine the material’s molecular composition and

structure.

• FTIR Analysis can be used to identify unknown materials, additives within polymers, surface contamination

on a material, and more.

• A simple device called an interferometer is used to identify samples by producing an optical signal with all

the IR frequencies encoded into it.

• The signal is decoded by applying a mathematical technique known as Fourier transformation. This

computer-generated process then produces a mapping of the spectral information. The resulting graph is

the spectrum which is then searched against reference libraries for identification.

• FTIR can also measure levels of oxidation in some polymers or degrees of cure in other polymers as well as

quantifying contaminants or additives in materials.

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FTIR – Working Principles• Michelson interferometer is the key component of the

FTIR system

• The infrared radiation from a source enters the

Michelson interferometer. The interferometer is

composed of one beam splitter and two mirrors. The

beam-splitter transmits half of the infrared (IR) beam

from the source and reflects the other half. The two

split beams strike a fixed mirror and a moving mirror,

respectively. After reflecting from the mirrors, the two

split beams combine at the beam splitter again in

order to irradiate the sample before the beams are

received by a detector.

• The moving mirror is used to change the optical path lengths to generate light interference between the

two split beams.

• If the moving and fixed mirrors, both are located at the same distance from the beam splitter optical

paths of the two split beams are the same zero path difference.

• To generate an optical path difference (δ) translation of moving mirror away from the beam splitter.

• Due to the change in the optical-path, the two split beams will show constructive and destructive

interference periodically, with continuous change of δ value. There will be completely constructive

interference when δ =nλ, but completely destructive interference when δ =(1/2 + n)λ.12

FTIR – Working Principles

• A plot of light interference intensity as a function of optical path difference is called an interferogram.

• The FTIR detector receives interferogramsignals that are transmitted through a sample (or reflected from a sample).

• The interferogram received by the detector is not an infrared spectrum.

• Fourier transformation is necessary to convert an interferogram into an infrared spectrum, which is a plot of the light intensity versus wave number.

• The infrared light sources: (i) Nernst Glower (oxides of rare earths), and (ii) Globar(composed of Silicon Carbide)

Plots of an interferogram (left) and its Fourier transform

from an interferogram to an IR spectrum (right)

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FTIR – Spectra

(a) Single-beam FTIR spectrum of background: a plot of raw detector response versus wave number without a sample; (b) a sample single-beam FTIR spectrum of polystyrene (vibrational bands of polystyrene are superimposed on background spectrum); and (c) final FTIR spectrum of polystyrene that only contains the vibration bands from the polystyrene sample (absorption intensity is expressed as the transmittance).

Transmittance, T= I/IoWhere I is the intensity measured in a single-beam spectrum of sample and Io is theintensity measured in the background spectrum.

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FTIR – Sample Preparation

• Solid samples for transmittance examination can be in one of two forms: thin film or powder.

• Thin-film samples are mainly polymeric materials. Casting films with polymer solutions is a commonly used method. Polymer films can also be made by mechanical pressing under elevated temperatures.

• Powder samples are made by grinding solid to powder and then diluting the powder with infrared-inert matrix materials. There are two typical methods for preparing powder samples: making KBr pellets and making mulls.

• The simplest method to prepare a liquid sample is to make a capillary thin film of the liquid. The capillary thin film is made by placing a drop of liquid on a KBr plate and sandwiching it with another KBr plate.

• Liquid cells can be used for volatile liquid and toxic liquid samples.• Cells for gas samples are structurally similar to cells for liquid but the dimension is much

larger.

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Fourier Transform Infrared Microspectroscopy

• A combination of FTIR spectroscopy with microscopy for generating FTIR spectra from microscopic volumes in materials.

• The instrument for FTIR microspectroscopy is called the FTIR microscope, which is often attached to the conventional FTIR instrument.

• The FTIR microscope is increasingly used for materials characterization because of its simple operation and FTIR spectra can be collected rapidly from microscopic volumes selected with the microscope.

• The FTIR microscope has an optical system that can easily switch between visible light observation and infrared light spectroscopy.

• The microscope can be operated in either transmittance or reflectance modes in order to meet the sample conditions, either transparent or opaque to light. The microscope has two light sources: a visible light source and an infrared light source. It also has an infrared detector and video camera or eyepieces, like a regular light microscope.

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Fourier Transform Infrared Micro-spectroscopy - Applications

Micrograph of isolated particulate contamination

(AM) and cotton fiber (C) IR spectrum of the isolated particle (lower) and IR

spectrum of polystyrene (upper)

An example of using the FTIR microscope to identify a microsized particle.17

Raman Microscopy or Raman Microspectroscopy• Capable of examining microscopic areas of materials by focusing the laser beam down to the

micrometer level with least sample preparation.• Generally, it is not used for imaging purposes, similar to FTIR microspectroscopy.• Its spatial resolution is at least 1 order of magnitude higher than the FTIR microscope.• It is of the dispersive type and requires collecting a spectrum at each wave number separately,

unlike the FTIR.• Its working principles are same as those of conventional dispersive Raman instruments,

consisting of a highly monochromatic laser source, a sample illumination and collection system, a spectral analyzer, and detection and computer control and processing system.

Commonly used laser sources

are gas continuous-wave lasers

such as Ar+, Kr+, and He–Ne.

Such laser sources are often

capable of generating beams of

multiple wavelengths.

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Raman Imaging• A technique to obtain spatial distribution of specific molecules in a sample,

similar to element mapping in X-ray, electron, and secondary ion mass spectroscopy.

• The working principle is to record the specific Raman peak that represents the specific component in the sample.

• It is much more difficult than other mapping techniques due to inherently weak Raman scattered light.

• It can be either obtained by a scanning or a direct-imaging method in the Raman microscope.

• A Raman image of a specific molecule is obtained by mapping its unique wave number of Raman scattering.

• Only the scattered light with a wave number should be recorded by a two-dimensional detector.

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Raman Spectroscopy/Microscopy - Applications

• Raman spectroscopy is an attractive technique for ceramic and polymeric

materials due to its simplicity by illuminating their surfaces regardless of sample

thickness and form.

• Raman microscopy is even better due to its ability to examine a microscopic

area with diameters in the order of 1 μm.

• Raman microscopy can be used for: phase identification of polymorphic solids,

polymer identification, composition determination, determination of residual

strain, and determination of crystallographic orientation.

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Raman - Phase Identification

• Its ability to determine phases for polymorphic solids at the microscopic level

has an advantage over conventional X-ray diffraction spectrometry in which the

sample volume cannot be too small.

• Phase identification with Raman spectroscopy uses the characteristic vibration

band(s) associated with a certain phase in a solid.

• For Example, four types of carbon spectra can be obtained that represent highly

oriented graphite, polycrystalline graphite, amorphous carbon, and diamond-

like carbon. Significant differences appear in Raman spectra for graphite and

diamond because carbon atoms are bonded with sp2-type orbitals in graphite,

while they are bonded in sp3-type orbitals in diamond.

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Raman - Phase Identification

Characteristic spectra of carbon in different structures: (a) highly oriented

graphite; (b) polycrystalline graphite; (c) amorphous carbon; and (d) diamond-like carbon.

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Raman - Polymer Identification• It is able to identify the different types of

polymers even though they all contain C, H, and O.

• For example, the Raman spectra of a laminated polymer film is able to identify individual layers by focusing the laser beam on each of the layers in the cross section of the laminated sample. The Raman spectra reveal the laminated sample is composed of polyester, polyethylene, and a layer of paper.

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Raman - Polymer Identification• Raman spectroscopy is sensitive to polymer conformation. • For example, a polymer blend of polybutadiene–

polystyrene in which polybutadiene is used to increase toughness of the polystyrene can be examined by Raman microscopy to identify its heterogeneity.

• Polybutadiene has three isomer conformations (cis-1,4, trans-1,4, and syndiotactic-1,2). These three types of isomers can be identified from C=C stretching modes.

• The Raman spectra of the copolymer indicate the difference in amounts of isomer types at the edge and thecenter of the polybutadiene–polystyrene sample.

Raman spectra of polybutadiene–polystyrene copolymer in the C=C stretching region showing three

distinct bands of polybutadiene isomers: cis-1,4 at 1650 cm-1 trans-1,4 at 1665 cm-1, and

syndiotactic-1,2 at 1655 cm-1 : (a) spectrum from the center of the sample and (b) spectrum from

the edge of sample. 24

Raman - Composition Determination• Monitoring the amount of chemical elements present

in, or added to, a solid.• The graphite spectrum changes according to the

number of molecules intercalated into its layered crystal structure.

• Molecular intercalation into gaps between the (0001) planes of graphite changes the bonding strength between the planes, which in turn changes the vibrational frequency in the graphite.

Raman spectra of graphite and graphite intercalation compounds (GICs) with

FeCl3. A lower stage number indicates a higher degree of intercalation. 25

Raman - Composition Determination

Raman spectrum of a Si1−xGex thin film with

thickness of 121nm on Si substrate, with argon

laser (457.9 nm) excitation. Note the Raman shift

of Si–Si vibration in Si1−xGex is different from that

of Si–Si in pure Si.

Raman peak shift relationship with the composition

change in Si1−xGex thin film. The plot also indicates

the film thickness effect on the relationship. Note

that the Raman shift position is linearly related to

the amount of Ge added in the Si.

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Raman - Determination of Residual Strain

Comparison of Raman peak position in

bulk Si and a Si film on sapphire (SOS)

• Residual strain in a solid can be determined by the shift of Raman bands.

• Strain changes the length of a bond between atoms.

• Vibrational frequency is exponentially affected by the bond length.

• For example, Compressive strain in a microscopic area of a sample will reduce the bond length increased corresponding vibrational frequency.

• The Si stretching vibration changes its frequency when Si is in the form of a thin film on sapphire.

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Raman - Determination of Crystallographic Orientation

• Crystallographic orientation in a sample area can also be determined by measuring

the intensity change of Raman scattered light in different polarization directions.

• When polarized light is scattered by a crystallographic plane of a sample, the

polarization of light will be changed.

• The degree of change will depend on the angle between the polarization direction of

incident light and direction of the crystallographic plane, and the angle between the

polarization direction of scattered light and direction of the crystallographic plane.

• By measuring the scattered-light intensity change with polarization direction using an

analyzer, the direction of the crystallographic plane at the examined sample area can

be determined.

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• Students can refer to the prescribed reference books on the topic for more detailed

understanding.

• Text Book: Materials Characterization by Yang Leng

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